1. Technical Field
This invention relates to acoustic charge transport devices and more particularly to an acoustic charge transport device in which the charge carrier density in the transport channel can be controlled.
2. Discussion
Acoustic charge transport devices such as heterojunction acoustic charge transport devices are devices which are used to propagate charge carriers through a transport channel formed in a piezoelectric semiconductor substrate. Such acoustic charge transport devices generally include an input ohmic electrode for delivering charge carriers to an electropotential well established within the transport channel, as well as an output ohmic electrode for receiving charge carriers after they have propagated through the transport channel. Finally, such acoustic charge transport devices often include a surface acoustic wave transducer which is able to generate a surface acoustic wave and deliver the wave to the transport channel. The charge carriers which are in the electropotential well within the transport channel are then carried to the output ohmic contact in response to propagation of the surface acoustic wave through the transport channel.
Tight control of the transport channel""s be precisely controlled charge carrier density is required for the acoustic charge transport devices so that the charge induced by the surface acoustic wave is sufficiently high as well as to maximize the signal-to-noise ratio. However, the charge carrier density is often a function of process-dependent as well as temperature dependent variations. With respect to process dependent variations, it will be noted that acoustic charge transport devices are often formed by molecular-beam epitaxy in which multiple molecular beams having different flux and chemistry are used in an ultrahigh-vacuum epitaxial growth process. If the doping or the thickness of the layers which form during this process are lower than certain particular ranges, the resulting charge carrier density within the transport channel is relatively small. As a result, a relatively high channel resistance is produced and a relatively small charge is induced in the transport channel by the surface acoustic wave. A similar result is obtained if the operating temperature of the acoustic charge transport device is lower than expected.
Furthermore, if the doping or the thickness of the layers of the acoustic charge transport device are greater than desired, the excess charge carriers will tend to accumulate within the electropotential well. This causes non-isolated wave packages to travel on the surface of the transport channel thereby producing a small signal-to-noise ratio. A similar result is obtained when operating temperatures are higher than expected.
In accordance with the teachings of the present invention, an acoustic charge transport device is provided. The acoustic charge transport device is formed by a process which introduces process dependent variations in charge carry density within the device. The acoustic charge transport device includes a transport channel which is operable to carry charge carriers in conjunction with the movement of a surface acoustic wave. In addition, the acoustic charge transport device includes means for controlling the charge carry density within the transport channel. By controlling the charge carrier density within the transport channel, the process dependent as well as temperature dependent variations in charge carry density within the acoustic charge transport device can be controlled.